The two parties just have some much of the stuff they have dominated the political machine in the US.
On a presidential level, I don't see a time in the near future whereby a 3rd party candidate has a snowball's chance in Hades.
The only place I can envision successful 3rd party candidates is the House of Reps, and maybe over time, the Senate.
Hell the only difference between a US election and a communist one is the choice of two parties instead of one.